They also reveal the political honours scrutiny committee, in charge of overseeing his appointment to the House of Lords, wanted to see proof that Ashcroft would become a full UK taxpayer before he took his seat.

But the proof never arrived, and after further submissions from the Tories, the deal was waved through.

On a day of fast-moving developments:

• Hague admitted he was wrong to declare Ashcroft would pay "tens of millions" of pounds more in tax as a result of the deal struck in 2000 to allow the Tory donor to take a seat in the Lords.

• Lady Dean, a member of the body that oversaw the peerage bid and a Labour peer, told a Commons committee that she was "shocked" to find out that Ashcroft was still not a full UK taxpayer. "It looks like the undertakings were not carried through," she said.

• Sir Hayden Phillips, the senior civil servant who shepherded the peerage through the appointments process, came under pressure to explain why he had allowed Ashcroft to enter the Lords through despite the honours committee's reservations.

Ashcroft revealed earlier this month that, despite becoming a UK resident under the deal, he remains "non-domiciled" for tax purposes, meaning he does not pay UK tax on his overseas earnings.

Today's developments have exposed the Tories – and Hague in particular – to further criticism over the affair.

Chris Huhne, the Liberal Democrat home affairs spokesman, told the BBC Hague was "completely unfit to be foreign secretary". He said the Tory frontbencher was complicit in "a cynical cover-up" with Ashcroft over his non-dom status.

The business secretary, Lord Mandelson, said the documents showed Hague and the Tory leader, David Cameron, had been "economical with the truth" and had not had "the courage to stand up to Lord Ashcroft" over his tax status.

The previously unpublished documents cast new light on the tortuous negotiations over Ashcroft's peerage 10 years ago.

In a letter on 12 April 2000, two weeks after Ashcroft's peerage was announced, the honours scrutiny committee – through its secretary, Sir Anthony Merifield – said the Tory donor should not be allowed to take up his seat in the Lords until he became fully resident in the UK.

In another letter, on 9 May, Merifield asked for evidence from Ashcroft of the Inland Revenue form that would confirm he had moved his tax affairs to the UK.

Later, Phillips appeared to accept the view of the then Tory chief whip, James Arbuthnot, that Ashcroft did not need to become domiciled to take up his peerage. Giving evidence to the Commons public administration select committee this morning, Dean told MPs: "It looks like the undertakings given were not carried through."

Dean was asked whether she had been shocked by the recent revelations over Ashcroft's tax status. She replied: "Yes. We thought that the undertakings that he had given ... were honoured."

Gordon Prentice, a Labour member of the committee, told Phillips at the hearing in Westminster today that he had accepted "hook, line and sinker" assurances from Arbuthnot that domicility was not an important issue.

Phillips said it was not his role to interpret Ashcroft's precise tax status. But he conceded that the final wording agreed between the honours committee and the Tories should have been clearer. "I acknowledge that was, in hindsight, a mistake in the process," he said.

Phillips conceded there had been no one to "police" Aschroft's undertakings to ensure "the full set of reassurances" were followed through.

In an interview with Radio 4's Today programme this morning, Hague admitted he was wrong to predict in a letter to Tony Blair that if Ashcroft became a peer it would "cost him and benefit the Treasury tens of millions of pounds a year in tax".

Hague said: "The one thing I will concede which I think in retrospect was a mistake was to say tens of millions, because it may have cost him millions, it may cost him millions in the future – we don't know.

"None of us can know other than him and the tax authorities what it has cost."

Hague repeated his insistence that he did not "go into the detail" of the arrangement at the time and said that – 10 years on – he had not been asked about the tax element "as far as anyone involved can recollect".

"I made clear to James that I was going to be satisfied with any agreement which implemented the original undertaking and which the government officials dealing with it and the honours scrutiny committee were happy with."

Mandelson seized on the developments. "I don't know what sort of hold Lord Ashcroft has over David Cameron that he could not find the backbone, the bottle, to ask the relevant questions. In my view, that says rather a lot about David Cameron."

Huhne said the Tory attitude was "tax is for the little people, and the big people, if they are giving enough money to the Conservatives, can be let off paying tax".

Ashcroft has given more than £4m to the Tories since Cameron became leader in 2005, directing the funds at the key marginal seats that the party needs to win to gain power.

The Independent reported today that a BBC Panorama investigation involving interviews with politicians and businessmen in Belize and the Turks and Caicos islands, where Ashcroft has business interests, had been put on hold.

A BBC spokesman told the Independent: "The programme is not on the schedule at the moment. We do not know when it will be aired." BBC sources said lawyers had become involved.